In recent years, much consideration has been given to solar cells (solar electricity generation), which can be used continuously, which does not cause exhaustion of resources, and which hardly causes environmental pollution. Solar cells are classified roughly into inorganic solar cells, such as Si type and non-Si type, and organic solar cells, such as dye-sensitization type and organic thin film type. Inorganic solar cells are generally high in photoelectric conversion efficiency, but they are disadvantageously high in production cost because high vacuum is required or heat treatment at high temperatures is required. With organic solar cells, since a film can be formed by a coating process, a printing process, etc., their production cost is lower and films large in area can be produced. Additional advantage of organic solar cells over inorganic solar cells is that the former can afford elements lighter in weight. Particularly, since organic thin film type solar cells are more suitable for a printing process and are easy to be formed into film and the like, it has been reported that the production of flexible solar cells is easy.
However, many organic solar cells are low in photoelectric conversion efficiency, improvement in photoelectric conversion efficiency has been considered as a challenge.
Examples of materials with which high photoelectric conversion efficiency with organic thin film type solar cells has been achieved include a bulk heterojunction made of a mixed material made up of a p-type organic semiconductive material P3HT [poly(3-hexylthiophene] and an n-type organic semiconductive material PCBM [[6,6]-phenyl-C61-butylic acid methyl ester] (see, for example, Non-Patent Literature 1). Although small molecules such as pentacene are used as p-type organic semiconductive materials in some cases, polymer type materials are generally believed to be more suitable for the manufacture of elements by coating and therefore they are believed to be easier to achieve reduction in cost and enlargement in screen size.
One feature sought with p-type organic semiconductive materials is that a material has therein a highly planar π conjugated plane. This is because higher π-π interaction or higher carrier transfer efficiency are expected, and accordingly, high photoelectromotive force can be provided.
Patent Literatures 1 to 3 disclose polymer-type p-type organic semiconductors.
However, improvement in photoelectric conversion efficiency has been desired therewith more than organic solar cells.